diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 19bce21..c5bdc9e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['2.7', '3.0']
+        ruby: ["3.1", "3.2", "3.3", "3.4"]
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
diff --git a/Gemfile b/Gemfile
index 4ae9835..6d4f691 100644
--- a/Gemfile
+++ b/Gemfile
@@ -20,3 +20,7 @@ gemspec
 gem 'rake'
 gem 'rdoc'
 gem 'activesupport'
+
+group :test do
+  gem 'mutex_m'
+end
diff --git a/lib/iq_rdf/document.rb b/lib/iq_rdf/document.rb
index 01d7d22..b0d503e 100644
--- a/lib/iq_rdf/document.rb
+++ b/lib/iq_rdf/document.rb
@@ -145,16 +145,16 @@ def to_ntriples
     def to_turtle
       s = ""
       @namespaces.values.sort{ |n1, n2| n1.turtle_token <=> n2.turtle_token }.each do |namespace|
-        s << "@prefix #{namespace.turtle_token}: <#{namespace.uri_prefix}>.\n"
+        s += "@prefix #{namespace.turtle_token}: <#{namespace.uri_prefix}>.\n"
       end
-      s << "\n"
+      s += "\n"
       @nodes.each do |subject|
         pref = subject.to_s(@document_language)
         indent = "".ljust(pref.length)
 
         # Render subject, if it is defined as a RDF-type.
         if subject.rdf_type
-          s << "#{pref} a #{subject.rdf_type}"
+          s += "#{pref} a #{subject.rdf_type}"
           pref = ";\n" + indent
         end
 
@@ -163,13 +163,13 @@ def to_turtle
           objects = predicate.nodes.map { |object| object.to_s(indent: indent, lang: predicate.lang || subject.lang || @document_language) }
                              .join(", ")
 
-          s << "#{pref} #{predicate.to_s} #{objects}"
+          s += "#{pref} #{predicate.to_s} #{objects}"
 
           pref = ";\n" + indent
         end
-        s << ".\n"
+        s += ".\n"
         if @config[:empty_line_between_triples]
-          s << "\n"
+          s += "\n"
         end
       end
       s